Conversion of hydrocarbon oils



Patented May 28, 1940 UNITED STATES PATENT OFFICE Charles H. Angell, Chicago, 111., assignor to Universal Oil Products Company, Chicago, 111., a

corporation of Delaw Application July 31,

1 Claim.

The invention relates to an improved process for the selective conversion of hydrocarbon oils of diiferent cracking characteristics and embodiesa separation of the intermediate liquid o arbitrary separation of the total intermediate products with respect to the boiling characteristics of the several fractions.

One of the most important major advances in the early cracking art is disclosed in the United States Patent No. 1,392,629 issued to C. P. Dubbs and resides in the separation of residue from, the cracked products and the return of only reflux condensate, comprising relatively clean intermediate products of the cracking operation, to further cracking within the system. This major improvement was rapidly and widely adopted by the industry and for some years its most widely used commercial embodiment involved the return of the total reflux condensate to further cracking in the same heating coil to which the virgin charging stock for the process was supplied.

Further improvements, such as disclosed in the Egloff Patent No. 1,537,593, which teaches cracking of the virgin charging stock and reflux condensate under independently controlled cracking conditions and Pollock Patent No. 1,658,415, which teaches separation of the reflux condensate into light and heavy fractions, each of which are cracked separately from the charging stock, have since come into such common use that, at the present time, practically all of the most successful high-capacity cracking installations employ some form of selective cracking. In some cases the virgin charging stock, as well as the intermediate liquid products of the process, are separated into selected relatively low-boiling and high-boiling fractions for separate conversion under independently controlled cracking condi-- tions, while in other cases only the charging stock or only the reflux condensate is so separated.

Selective cracking, such as above mentioned, is based on the dependable theory that with oils of relatively wide boiling range from a common source, the low-boiling fractions require more drastic cracking treatment than the high-boiling fractions in order to produce the optimum yields of good quality gasoline. -In applying this theory it has become an almost universal practice are 1937, Serial No. 156,747

to separate the charging stock and/or the intermediate liquid conversion products, or both, into selected fractions of different boiling range characteristics and to subject each of said selected fractions to independently controlled cracking conditions.

I have found that, although the theory of selective cracking, as above stated, is dependable, its commercial adaptations are often not in strict accordance with the theory and that best results cannot be obtained by arbitrarily separating the total reflux condensate produced in a selective cracking operation into fractions of different boiling range characteristics and cracking the total intermediate products of the same boiling range under identical conditions. This is due to the fact that cracking of the intermediate products resulting from initial cracking of the virgin oil produces other intermediate products which, although they may be within the same boiling range as the first mentioned intermediate products, possess different cracking characteristics and usually require more drastic treatment to produce the best yields of gasoline. It also follows that the second recycle stock cracks to produce intermediate products of still different characteristics. This is not contrary to the theory of selective cracking, as above stated, since the source of the intermediate products is the cracking operation wherein they are produced and each cracking step produces somewhat different intermediate products than the preceding step. Theoretically, therefore, it is not only advantageous to crack the intermediate products resulting from initial conversion of the charging stock under different conditions than those to which the charging stock is subjected, but it is also advantageous to crack the intermediate products of the second cracking operation under still different conversion conditions, and so on, ad infiniturn. This may be termed multiple-stage cracking, as distinguished from the usual con-.- cept of selective cracking. Obviously, there are definite practical limits: in the application of this principle beyond which the beneficial results obtained are not proportionate to the additional cost of the installation and additional operating expense. The same applies to the selective cracking of intermediate fractions segregated according to their boiling characteristics and the extent to which the theory of either selective cracking or multiple-stage cracking can be utilized to practical advantage depends to a large extent upon the capacity of the installation. With most of the modern high-capacity 55 cracking installations, it is necessary to employ a plurality of heating coils, reaction chambers, flash distilling or coking chambers, fracticnaters and the like in order to keep the size of the individual pieces of equipment within economical limits and, when duplicate equipment of this kind is required, due to high capacities, it is distinctly advantageous to employ some form of selective or stage cracking. The present invention is intended to meet these particular requirements by providing an improved process which combines the principles of both selective cracking and multiple-stage cracking in a simple system which involves little or no duplication of equipment other than that necessitated for best economy in a high-capacity cracking system.

The present invention is best adapted to the treatment of a virgin charging oil of relatively narrow boiling range such as kerosene distillate, gas oil, fuel oil, or the like as distinguished from crude petroleum, topped crude or other oils of wide boiling range. However, the invention is not limited to the use of any particular type of charging stock, since it will also be found advantageous for the treatment of wide boiling range oils.

One specific embodiment of the invention comprises subjecting virgin charging stock for the process to cracking conditions of elevated temperature and superatmospheric pressure in a heating coil and communicating enlarged reaction chamber, separating vapors and non-vapor ous residue from the resulting conversion products, fractionating said vapors to condense their components boiling above the range of gasoline and separate the same into relatively lo\vboiling and relatively high-boiling reflux condensates, subjecting said relatively high-boiling reflux condensate to independently controlled conversion conditions of cracking temperature and superatmospheric pressure in a separate heating coil, introducing the heated products from said separate heating coil into said reaction chamber, subjecting said relatively low-boiling reflux condensate in another separate heating coil and communicating enlarged reaction chamber to conversion conditions of cracking temperature and superatmospheric pressure controlled independently of and more severe than those employed for said cracking of the relatively highboiling reflux condensate, separating vapors and non-vaporous residue from the products of the said relatively severe cracking operation, fractionating the last mentioned vapors separate from those first mentioned to condense therefrom a second relatively heavy and a second relatively light reflux condensate, subjecting said second light and heavy reflux condensates each in another separate heating coil to conversion conditions of cracking temperature and superatmospheric pressure controlled independently of. and more severe than those employed for said cracking of the first mentioned light reflux condensate, supplying heated products from the last mentioned heating coils to said separate reaction chamber, subjecting fractionated vapors of the desired end-boiling point from both fractionating steps to condensation and recovering the resulting distillate.

It will be apparent from the above described specific embodiment of the invention that the process utilizes two separate but interdependent cracking systems, each employing separate heating coils, an enlarged reaction chamber and separating and fractionating equipment. The first of these systems is devoted to the treatment of the virgin charging stock and the high-boiling reflux condensate formed in the iractionating step of this system. The second system is devoted to the treatment of low-boiling reflux condensate from the fractionating step of the first cracking system and both low-boiling and highboiling reflux condensates from the fractionating step of the same (second) cracking system. This particular arrangement provides a high degree of selectivity both with respect to the independent cracking of oils of different boiling range characteristics and with respect to the independent cracking of recycled stock produced in different cracking steps of the system and represents a combination of selective and multiple stage cracking which obtains the benefits of both type of operation in a relatively simple and ellicient system.

The above outlined specific embodiment of the invention will be found most economical and advantageous as applied to high charging stock capacities of the order of 2000 to 5000 barrels or more per stream day. For best economy with lower capacities or when, for any other reason, it is desirable to reduce the initial cost of the installation, the invention provides an alternative arrangement wherein the heavy reflux condensate produced in the fractionating step of the first cracking system may be returned for further cracking to the same heating coil to which the virgin charging stock is supplied or wherein the heavy reflux condensate produced in the fractionating step of the second cracking system may I':

be retiuned for further cracking to the same heating coil to which the relatively low-boiling reflux condensate from the fractionating' step of the first cracking system is supplied, thus eliminating one heating coil in either of the cracking systems.

When, in accordance with the last mentioned embodiment of the invention, the virgin charging stock and the high-boiling reflux condensate produced in the fractionating step of the first cracking system are cracked in the same heating coil, the virgin charging stock is preferably an oil of somewhat lower boiling characteristics than said high-boiling reflux condensate, the boiling characteristics of the latter being controllable by varying the conditions of fractionation employed in the first cracking system.

When, also in accordance with the last mentioned embodiment of the invention, the relatively low-boiling reflux condensate from the fractionating step of the first cracking system and the relatively high-boiling reflux condensate from the fractionating stop of the second cracking system are cracked in the same heating coil, the latter is preferably an oil of somewhat higher boiling characteristics than the former and may be varied to suit requirements by control of the fractionating conditions employed in the second cracking system.

The accompanying diagrammatic drawing illustrates one specific form of apparatus embodying the features of the invention and wherein the process of the invention may be conducted.

Referring to the drawing, the raw oil charging stock is supplied through line I and valve 2 to pump 3 by means of which it is fed through line 4 and valve 5 to heating coil 6 which is disposed within furnace 1 and wherein the oil is heated to the desired cracking temperature, preferably at a substantial superatmospheric pressure, the

heated products being directed through line 8 and valve 9 into reaction chamber l0.

Chamber ill is also preferably maintained at a substantial superatmospheric pressure and is preferably insulated, although insulation is not indicated in the drawing, whereby the heated products supplied to this zone are subjected therein to continued cracking. In the particular case here illustrated, both vaporous and liquid conversion products are withdrawn in commingled state from the lower portion of the reaction chamber, after passing through substantially the entire length of this zone, and are directed through line I l and valve i2 into vaporizing and separating chamber l3.

It is also within the scope of the invention, when desired, to separate vaporous and liquid conversion products in chamber it and to sep arately remove the same therefrom, in which case said liquid products may be directed all or in part, by well known means, not illustrated, to cooling and storage as a final residual product of the process or they may be supplied through line H and valve i2 to chamber !3 for further treatment therein, as will be presently described. All or a portion of the vaporous products may be separately removed from chamber ill at any desired point in this zone and directed therefrom, by well known means, not illustrated, either to chamber !3 or, all or in part, directly to fractionator i8.

Chamber i3, when this zone is utilized, is pref erably operated at a substantially reduced pressure relative to'that employed in chamber In, by means of which appreciable further vaporization of the liquid products supplied thereto is accomplished. Separation of vapors and non-vaporous residue is also accomplished in chamber l3 and the latter may either be reduced to substantially dry coke in this zone, in any well known manner, or withdrawn therefrom as residual liquid through line i 4 and valve 85 to cooling and storage or, when desired, to any suitable coking equipment, not illustrated. The vapors evolved in chamber 5 3, as well as any vaporous products supplied to this zone from chamber iii, are directed from the upper portion thereof through line l6 and valve ii to fractionation in fractionator E8.

The components of the vapors supplied to fractionator is which are of a higher boiling nature than the desired light distillate product of this stage of the system, which distillate preferably comprises gasoline, are condensed within the fractionator as reflux condensate and the reflux condensate is separated into selected relatively low-boiling and relatively high-boiling fractions which are separately subjected to further treatment, as will be later described.

, Fractionated vapors of the desired end-boiling point are removed together with gaseous products from the upper portion of fractionator l8 and directed through line 19 and valve 20 to condensation and cooling in condenser 2!, the resulting distillate and uncondensed gases pass through line 22 and valve 23 to collection and separation in receiver 24. The uncondensed gases may be released from the receiver through line 25 and valve 25. Distillate is withdrawn from the re ceiver through line 21 and valve 28 to storage or to any desired further treatment.

When desired regulated quantities of the distillate collected in receiver 2 3 may be recirculated, by well known means, not illustrated, to the upper portion of fractionator is to serve as a cooling and refluxing medium in this zone.

Relatively high-boiling reflux condensate formed in fractionator I8 is withdrawn from. the lower portion of this zone and directed through line 29 and valve 30 to pump 3| by means of which it is fed through line 32 and may be supplied therefrom, as desired, either through line 33, valve. 34 and line 4 into heating coil 6, for further cracking in this zone in commingled state with the fresh charging stock, or it may be directed through valve 35 in line 32 to a separate heating coil 36 disposed within furnace 31.

When heating coil 36 is employed, relatively high-boiling reflux condensate supplied thereto is subjected in this zone to cracking conditions of elevated temperature, superatmospheric pressure and conversion time controlled independently of those employed in heating coil 6 for cracking the charging stock. These cracking conditions may be somewhat more or somewhat less severe than those employed in heating coil 6 or may be substantially the same, depending upon the relative cracking characteristics of the charging stock and the heavy reflux condensate.

The conversionv products formed in heating coil 36 are directed from this zone through line 38 and valve 39 into reaction chamber in and are subjected in this zone to further cracking treatment under the conditions prevailing therein, together with the products supplied to this zone from heating coil 6, the resulting vaporous and liquid products being subjected to the same subsequent treatment previously described.

The relatively low-boiling reflux condensate formed in fractionator i8 is withdrawn from one or'a plurality of suitable intermediate points in this zone and directed through line ll] and valve 4! to pump 42 by means of which it is supplied through line 43 and valve M to further cracking in heating coil 45.

Heating coil 45 is disposed within a suitable furnace 46 by means of which the light reflux condensate from fractionator i8 is subjected therein to the desired cracking temperature, superatmospheric pressure and conversion time. The cracking conditions employed in heating coil 45 are controlled independently of those employed in the other heating coils of the system and are pref erably more severe than those utilized in heating coils 6 and 36. The heated products formed in heating coil 45 are directed, in the case here illus trated, from this zone through line H and valve 48 to reaction chamber 59.

Chamber 49, like chamber Ill, is preferably operated at a substantial superatmospheric pressure and is preferably insulated to conserve heat, the heated products supplied to this zone being subjected therein to continued cracking under the temperature and pressure conditions prevailing in this zone. Both vaporous and liquid conversion products, in the particular case here illustrated, are withdrawn in commingled state from the lower portion of chamber 49, preferably after passing through substantially the entire length of this zone, and are directed therefrom through line 52! and valve 5! to vaporizing and separating chamber 52.

Separation of vapors and non-vaporous resi due is accomplished in chamber 52 and this zone is preferably operated at a substantially reduced pressure relative to that employed in chamber 49, by means of which appreciable further vaporization of the liquid products supplied thereto is accomplished. The remaining non-vaporous residue may be reduced in chamber 52 to substantially dry coke by any well known means or may be withdrawn from the lower portion of this zone as residual liquid and directed therefrom through line 53 and valve 54 to cooling and storage or, when desired, to suitable coking equipment, not illustrated. Vapors evolved from the liquid. products in chamber 52, as well as conversion products supplied to this zone in vaporous state, are removed from the upper portion thereof and directed through line 55 and valve 56 to fractionation in fractionator 57.

The components of the vaporous products supplied to fractionator which are of a higher bofling nature than the desired overhead disiillate product of this stage of the process are condensed by fractionation as reflux condensate and separated into selected relatively low-boiling and high-boiling fractions which are each subjected to separate further treatment within the system, as will be later described.

Fractionated vapors of the desired end-boiling point are removed together with gaseous products from the upper portion of fractionator 57 and may be supplied therefrom through line 56, valve 56 and line ES to condensation and cooling in condenser 2| and subsequent separation and recovery, as previously described, or, when desired, separate condensing and collecting equipment, not illustrated, may be provided for the vaporous products from fractionators l8 and 57.

Distillate from receiver 24 may, when desired, be recirculated in regulated quantities to the upper portion of fractionator 5'! to serve as a cooling and refluxing medium in this zone or, in case the light distillate product from fractionator 57 is recovered in a separate receiver, distillate from the latter zone may be utilized as a cooling and refluxing medium in fractionator 57.

Heavy reflux condensate formed in fractionator 5? is directed from the lower portion of this zone through line 66 and valve 6! to pump 62 by means of which it is fed through line 63 and may be directed therefrom, as desired, either through line 64, valve 65 and line 43 to heating coil 45, wherein it is further cracked in commingled state with the lower-boiling reflux condensate supplied to this zone from fractionator [8, or it may be directed through valve 66 in line 63 to a separate heating coil 61 wherein it is aiforded separate cracking treatment.

Heating coil 6'! is disposed within a suitable furnace 68 by means of which the heavy reflux condensate from fractionator 51 is heated under conditions of cracking temperature, superatmospheric pressure and conversion time, controlled independently of those employed in any of the other heating coils of the system. The cracking conditions employed in heating coil 61, when this zone is utilized, may be somewhat more or somewhat less severe than those employed in heating coil 45 or may be substantially the same, depending upon the relative cracking characteristics of the light reflux condensate from fractionator i8 and the heavy reflux condensate from fractionator 5?. The heated products formed in heating coil 61 are directed therefrom through line 59 and valve 76 to reaction chamber 46 wherein they are afforded additional cracking time in this zone under the conditions prevailing therein and in commingled state with the heated products supplied to this zone from heating coil 45.

The relativel light reflux condensate formed in fractionator 51 is withdrawn from one or a plurality of suitable intermediate points in this zone and directed therefrom through line H and valve [2 to pump '13 by means of which it is fed through line 14 and valve I5 to heating coil '16.

Heating coil 76 is disposed within a suitable furnace T! by means of which the light reflux condensate from fractionator 5'! is subjected to the desired conversion conditions of cracking temperature, superatmospheric pressure and conversion time, controlled independently of those employed in any of the other heating coils of the system and preferably more severe than those employed in any of the other heating coils. The heated products formed in heating coil 16 are directed therefrom through line 78 and valve 79 to reaction chamber 49 wherein they are afforded additional cracking treatment under the conditions prevailing in this zone and in commingled state with the other heated products supplied thereto as previously described.

Instead of supplying the total heated products from heating coils 45, 76 and from heating coil 61, when the latter zone is employed, to reaction chamber 49, one or more of the streams of heated products from these various heating coils or a regulated portion of any or all of them may, when desired, be supplied directly to vaporizing and separating chamber 52, by Well known means, not illustrated. In such cases the material thus supplied to chamber 52 may be cooled prior to their introduction into this zone, by any well known means, not illustrated, or may be supplied to this zone without intentional cooling. When not previously cooled, any heated products directly from any or all of the coils 45, 61 and 16 t0 chamber 52 may serve as a means of assisting coking of the residual products in this zone.

It is also within the scope of the invention to separate vaporous and liquid conversion products in chamber 49, in which case, this zone may be operated in the manner previously mentioned in the description of chamber H], or, when desired, chamber 49 may be eliminated and the products from heating coils 45, 16 and heating coil 61, when the latter zone is utilized, may be supplied directly to chamber 52, by well known means, not illustrated, either with or without intentional cooling of any or all of the various streams of heated products prior to their introduction into chamber 52.

The above mentioned alternatives serve to indicate only some of the various modifications of the specific flow illustrated in the accompanying drawing which may be employed without departing from the scope of the invention. Other modifications and minor departures will be apparent to those skilled in the art.

The operating conditions which may be employed in conducting the process of the invention in an apparatus such as illustrated and above described, are Within the following approximate ranges: The cracking conditions employed in heating coil 6 will, of course, vary considerably depending upon the type of charging stock utilized. With a relatively heavy charging stock, such as fuel oil, topped crude or other residual liquid, the temperature employed in heating coil 6 may range for example from 800 to 950 F., or thereabouts, with a superatmospheric pressure at the outlet of the heating coil of from 100 to 350 pounds or more per square inch. With a relatively low-boiling charging stock, such as, for example, kerosene distillate, gas oil or the like, more severe cracking conditions are preferably employed in heating coil 6, the temperature at the outlet from this zone ranging for example from 900 to 980 F. or more,

with a superatmospheric pressure at the outlet of the heating coil of from 200 to 500 pounds or more per square inch.

When separate cracking treatment of the reflux condensate from fractionator I8 is provided by the use of heating coil 36, the temperature employed at the outlet from this zone may range for example, from 850 to 950 F., preferably with a superatmospheric pressure at the outlet of the heating coil of from 150 to 500 pounds or thereabouts per square inch. Ordinarily the heavy reflux condensate formed in fractionator I8 will be of a higher boiling nature than the charging stock and it is ordinarily subjected to cracking conditions more severe than those employed in heating .coil 6 or substantially the same, except when the charging stock is an oil of lowerboiling characteristics than gas oil, for example, or when it is an oil requiring drastic cracking conditions, such as, for example, a parafiinic distillate or the like.

Reaction chamber I0, as'previously mentioned, is preferably operated at a substantial superatmospheric pressure which may range for example from 100 to 500 pounds or thereabouts per square inch and may be substantially the same or somewhat lower than that employed in the communicating heating coil utilizing the lowest pressure. When chamber I3" is utilized, this zone is preferably operated at a substantially reduced pressure relative to that employed in chamber ill, the reduced pressure ranging, for example, from 150 pounds or thereabouts per square inch down to substantially atmospheric pressure. Fractionator H3 in the succeeding condensing and collecting portions of the system may utilize pressures substantially the same or lower than the presure employed in chamber The light reflux condensate supplied from fractionator [8 to heating coil 45 may be sujected in this zone to cracking temperature, measured at the outlet therefrom, of from 925 to 1000 F. or thereabouts, preferably with a superatmospheric pressure at this point in the system of from 200 to 800 pounds or more per square inch.

When the heavy reflux condensate from fractionator 5'! is subjected to separate cracking treatment in heating coil 61, the latter zone may employ conversion conditions within substantially the same range as those above specified for heating coil 5, although the same cracking conditions are not necessarily employed in the two heating coils Q5 and the heavy reflux condensate from fractionator 51 being subjected either to substantially the same or to more severe or less severe cracking conditions than those employed in heating coil 45.

Heating coil 76, wherein the light reflux condensate from fractionator 51 is subjected to separate cracking treatment, preferably employs more severe cracking conditions than those utilized in any of the other heating coils of the systern, the temperature at the outlet from this zone ranging for example from 950 to 1050 F., preferably with a superatmospheric pressure at this point in the system of from 250 to 1000 pounds or thereabouts per square inch.

Reaction chamber 49, when employed, is preferably operated at a substantial superatmospheric pressure ranging, for example, of from 100 to 500 pounds or more per square inch and the pressure employed in this zone may be substantially the same or somewhat lower than pressure relative to that employed in chamber 40. When chamber 49 is not utilized, the pressure employed in chamber 52 preferably is substantially lower than that employed in the communicating heating coils. In either case, the pressure conditions employed in chamber 52 may range, for example, from 150 pounds or thereabouts per square inch, superatmospheric, down to substantially atmospheric pressure.

Fractionator 5i and the communicating condensing and collecting equipment may employ pressures substantially the sameor lower than the pressure employed in chamber 52.

As an example or one specific operation of the process, conducted in accordance with the provisions of the invention'in an apparatus such as illustrated and above described: The charging stock is'a Mid-Continent fuel oil of about 25 A. P. I. gravity which is subjected in heating coil 6 to an outlet temperature of approximately 890 F. at a superatmospheric pressure of about 300 pounds per square inch and thence discharged into reaction chamber l0, which is maintained at substantially the same pressure. The heavy reflux condensate from iractionator i0 is subjected in heating coil 30 to an outlet temperature of approximately 910 F. at a superatmospheric pressure of approximately 350 pounds per square inch and the heated products from this zone are also supplied to reaction chamber l0. Both vaporous and liquid conversion products are directed from reaction chamber it) to chamber l3 which is operated at a superatmospheric pressure of about 75 pounds per square inch and Whereirom a residual liquid product is recovered, the vapors from this zone being directed to fractionation in fractionator l8. The light reflux condensate from fractionator i0 is heated in heating coil 45 to an outlet temperature of approximately 950 F. at a superatmospheric pressure of about 350 pounds per square inch. The heavy reflux condensate from fractionator til is heated in coil ii! toan outlet temperature'oi approximately 930 F. at a superatmospheric pressure of approximately 350 pounds per square inch and the light reflux condensate from fractionator Ell is heated in coil 18 to an outlet conversion temperature of approximately 975 F. at a superatmospheric pressure of about 375 pounds per square inch. The heated products from heating coils 45, 67 and iii are supplied to reaction chamber d9, which is operated at a superatmospheric pressure of approximately 350 pounds per square inch, and both vaporous and liquid conversion products are directed from this zone into vaporizing and separating chamber 52 Which is operated at a superatmospheric pressure of approximately 100 pounds per square 'inch.

Both fractionating steps of the system are operated at a superatmospheric pressure of approximately 70 pounds per square inch and this pressure is substantially equalized in the succeeding condensing and collecting portions of the system.

The above described operation will produce per barrel of charging stock, approximately 63% of 400 F. endpoint gasoline of high antiknock Value and approximately 29% of liquid residue which is suitable for sale as premium fuel oil, the remainder being chargeable principally to uncondensible gas.

I claim as my invention:

A process for the pyrolytic conversion of hydrocarbon oils which comprises heating a virgin charging oil to cracking temperature at superatmospheric pressure in a heating coil, supplying the heated products to an enlarged reaction chamber wherein their pyrolytic conversion is continued, separating vapors and non-vaporous residue from resulting products, fractionating the vapors to form a relatively low-boiling and a relatively high-boiling reflux condensate, heating the relatively high-boiling reflux condensate to an independently controlled cracking temperature at superatmospheric pressure in a separate heating coil, supplying the heated products from said separate heating coil to said reaction chamber, heating the relatively low-boiling reflux condensate to an independently controlled higher cracking temperature at superatmospheric pressure in another separate heating coil, supplying the heated products from thelast mentioned heating coil to an enlarged reaction chamber separate from that first mentioned and therein subjecting the same to continued cracking, separating vapors and non-vaporous residue from the products of the last mentioned cracking operation, separately fractionating the last mentioned vapors to form another relatively highboiling reflux condensate and another relatively low-boiling reflux condensate, heating the last mentioned relatively high-boiling reflux condensate at superatmospheric pressure in another separate heating coil to an independently controlled cracking temperature and introducing the resulting heated products into said separate reaction chamber, heating the last mentioned relatively low-boiling reflux condensate at superatmospheric pressure in another separate heating coil to a higher cracking temperature than that employed in any of the aforementioned heating coils of the system and introducing the resulting heated products into said separate reaction chamber, subjecting fractionated vapors of the desired end-boiling point from both fractionating steps to condensation and recovering the resulting products.

CHARLES H. ANGELL. 

